1. Field of the Invention.
This invention relates to a mold member used in the precision casting of metals for dental prosthetic use and capable of delaying and preventing the solidification of the molten metal staying in the mold member called a sprue, runner, gate and molten metal reservoir adapted for feeding the metal into the mold cavity before the casting cools and solidifies.
2. Prior Art
It was a general practice to employ a lost-wax process in the conventional precision casting of dental metals (either centrifugal casting or pressure casting, in which the former is used in the description of the invention that follows) because of the smallness of the articles to be cast.
According to the lost-wax process, recourse is had to centrifugal casting which is carried out in such a manner that a casting and a sprue, runner, gate and, if necessary, a molten metal reservoir are produced from wax, the mold members thus produced are set in a ring filled with an investment compound, thereafter are heated at temperatures of 800.degree. to 900.degree. C. to burn the wax to make hollow the members corresponding to the wax, and molten metal for a casting such as a nickelchromium alloy (meltable at 1200.degree.-1300.degree. C.) is poured into the portions corresponding to the members thus made hollow by melting of the wax.
In this case, when the sprue, runner, gate and molten metal (which will hereinafter be referred to generically simply as a mold member) cool and solidify before the portion or portions corresponding to the casting cool and solidify, it becomes impossible to supply the portion or portions with molten metal from the mold member, and consequently when the portion or portions solidify, the casting is deformed by shrinkage or shrinkage cavities are produced in the casting because of shrinkage to thereby render it impossible to obtain an intended proper dental casting.
Furthermore, in the case of the dental casting, the casting is small in size, so that unless by centrifugal pressure (pressure applied by a pressure plunger in the case of pressure casting), pouring of molten metal into the mold member was impossible and pressure resistance of the mold member was also required. When pressure resistance of the mold member is not sufficient, the member may break down or may be deformed to thereby exert adverse effects on the substantial supply capability of molten metal.
Furthermore, since the casting is small in size, the sprue, runner, and the like must also be made small, but because cooling and solidifying of the casting is quick, size reduction in a mold member is impossible, with the ultimate result that the mold also cannot but be increased in size. A detailed description will now be given of a conventional mold with reference to a known embodiment thereof.
FIG. 1 is a longitudinal sectional view of a centrifugal casting mold for use in a conventional bridge, the view showing the case wherein three connected teeth having supporting crowns b on either side of a pontic a in the middle are cast.
The pontic a is made of a mass block having thickwalled portion, and supporting crowns b on both sides of the pontic a are thin-walled. Because there is a great difference in thickness between the pontic and the crowns, it is a general practice to cast them as shown by pouring molten metal through three branched gates c. In this case the thin-walled supporting crowns b on both sides are the first, the gate c is the second, the runner d the third, the sprue e the fourth and the thick-walled pontic a is the last in order in which the molten metal is cooled and solidified. As a result, when molten metal in the pontic a is solidified, it shrinks inwardly. On the other hand, since the molten metal in the sprue e, runner d, and gates c has already been solidified, it cannot find its way through them, so that shrinkage cavities and pores are produced in the pontic a. The defective structure mentioned above produces the following adverse effects.
(i) The presence of cavities makes it difficult for the food pieces left in the mouth to be removed from the cavities, which fact is not desirable for oral hygiene; PA1 (ii) The presence of cavities weakens a casting and makes it liable to breakage; PA1 (iii) The presence of cavities hinders bonding of porcelain with a ceramic material in fusing the porcelain over the material and in firing and produces foams, leading to fatal defects; PA1 (iv) The presence of cavities reduces aesthetic value of an article; and PA1 (v) The presence of cavities reduces dimensional accuracy because of the deformation brought about by shrinkage.
In consideration of the drawbacks described above, it is a general practice to enlarge mold members such as a sprue, runner in order to delay the solidification of molten metal in the mold members, but the practice has various disadvantages. Namely, when the sprue, runner and gates are enlarged, solidification and shrinkage of the molten metal therein are increased in degree to deform a casting and make it difficult to cut and finish gate portions, and thus reduce dimensional accuracy and also make it necessary to use molten metal for costly dental metals in large quantities.
The sprue varies depending upon the dimensions and volume of a casting, and is usually designed to have a diameter of the order of 2.5 to 5 mm. Particularly, when it is desired to make a thick-walled casting, it is necessary to use a sprue having a separate molten metal reservoir or make a closed molten metal reservoir to feed molten metal from the molten metal reservoir to the shrunk portion of the thick-walled portion to compensate for an amount of shrinkage due to delayed cooling of the thick-walled portion by the use of a sprue having a separate molten metal reservoir or by providing for a closed molten metal reservoir. Thus, the sprue becomes complicated in structure and the use of molten metal must also be increased in quantity.
With respect to a casting different in thickness, recourse is had to quenching caused by a chill effect by using a cooling metal in a portion corresponding to the thick-walled portion, but this is not suitable for precision casting for use in dentistry from the viewpoint of the maintenance of dimensional accuracy.